Gas combustion engine, and method of manufacturing the gas combustion engine

ABSTRACT

In a gas combustion engine, which is manufactured by conversion of a diesel engine, the piston shape, the compression ratio, the air intake system and the exhaust gas system are modified. The exhaust gas system is split into two parts, each of which consists of an exhaust branch-piece and a catalyser, whereby catalysers of standard format can be employed, and the air intake system is made smaller, whereby the probability of backfire is reduced. The hollow in the piston already present in the diesel engine is enlarged in diameter and depth, in such a manner that the ratio of the diameter to the depth is equal to 30:11. In this manner, one achieves an optimal form with respect to heat extraction. In addition, enlargement of the hollow causes a lowering of the compression ratio to a value of 1:10, which is necessary when applying gas as a fuel. In addition, the edge of the piston near the top surface of the piston is bevelled, in order to compensate for extra expansion of the piston around the hollow, which extra expansion occurs as a result of the increased combustion temperatures. The costs of the above-specified conversion of a diesel engine to a gas combustion engine are hardly higher than those of a revision service of a diesel engine, so that there is no longer a financial obstacle to conversion to gas as a fuel.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a gas combustion engine comprising a gasexhaust system, an air intake system, an engine block comprising anumber of cylinders and pistons, which pistons can be translated in thecylinders, and a cylinder head which delimits the space in the cylindersat one extremity, whereby the volume of the space in a cylinder betweenthe cylinder head and a piston can be varied between a minimal and amaximal value by means of the translatory motion of the piston, and theratio of the said minimal and maximal values defines a compressionratio, and a combustion space is defined by the space in that state ofthe piston which corresponds to a minimal volume. The invention alsorelates to a method of manufacturing such a gas combustion engine.

2. Description of the Related Art

Trucks and buses in Europe are fitted standard with diesel engines, foreconomic reasons (power, lifetime, fuel consumption). An alternative fordiesel engines is formed by gas combustion engines running on LPG ornatural gas. Disadvantages of gas combustion engines, however, are thatthey have a lower power and a higher fuel consumption than dieselengines. In addition, gas combustion engines are more expensive thandiesel engines. This is caused inner alia by the fact that gascombustion engines are not yet manufactured by mass productiontechniques, and by the fact that the trucks and buses are deliveredstandard from the factory with diesel engines, so that modification isrequired if such a vehicle is to be made suitable for incorporation of agas combustion engine.

An increasingly important disadvantage of diesel engines, however,concerns their pollution of the environment with exhaust gases. Suchpollution is very much greater in the case of diesel engines than in thecase of gas combustion engines. Gas combustion engines are generallyknown to be relatively clean engines, which pollute the environment toonly a slight extent with exhaust gases which are much cleaner thanthose produced by diesel engines. This disadvantage of diesel engines isof even greater significance in the case of transport in denselypopulated areas, particularly in cities. In such areas, thedisadvantages of the said pollution are felt to a greater extent thanoutside cities.

In order to alleviate the problem with respect to pollution,ever-increasing numbers of vehicles for intended use in city traffic arebeing fitted with gas combustion engines. Such vehicles arepredominantly new ones, which are used to replace older, written-offvehicles, and are provided with new gas combustion engines duringproduction; alternatively, the new gas combustion engine in suchvehicles is provided during a revision service, and is a completereplacement for a former written-off diesel engine in the vehicle. Inboth instances, special modifications to the vehicles are necessarybefore the gas combustion engines can be fitted.

However, since the economic lifetime of diesel engines is long, which,for financial reasons, tends to be a dissuading factor in possible earlyreplacement of the diesel engines, a large percentage of all suchvehicles will remain fitted with diesel engines in the coming years,even if all new vehicles and revised vehicles were to be fitted with gascombustion engines.

An object of the invention is the provision of a financially attractivesolution to the problem of pollution hereabove elucidated. To this end,the gas combustion engine in accordance with the invention ischaracterised in that the gas combustion engine is a converted dieselengine, whereby the combustion space and the compression ratio areadapted to application of gas as a fuel, whereby the pistons have acylindrical wall and a top surface, whereby a hollow is located in thetop surface, which hollow forms part of the combustion space and has anapproximately circle-cylindrical form and an approximately flat bottomand the hollow has a diameter and a depth, whereby the ratio of thediameter to the depth lies between 30:9 and 30:13. The diesel engine ispreferably converted to a gas combustion engine during a revisionservice.

From WO 91/14086 the conversion from a diesel engine into a gas engineis known per se, but

without reducing the compression ratio;

without controling and varying the combustion point between 12° and 20°before TDC (top dead center) and

without controlling the gas/air ratio using lambda probes and an engineregulation unit.

Consequently this known engine has a short life expectation and a highpollution of the environment. The diesel engine is preferably convertedto a gas combustion engine during a revision service. The diesel engineis not replaced by an other engine, so that no (or, at least,practically no) modifications need be made to the vehicle. Above all,most of the diesel engine remains suitable for use as a gas combustionengine without any modification, as a result of which the depreciationcosts are affected to a minimal extent (or even not at all). The costsof such a conversion are only slightly higher than the costs of revisionof the diesel engine, so that there are hardly any reasons for notconverting the diesel engine to a gas combustion engine during arevision of the former. Above all, it has transpired that the power andlifetime of such gas combustion engines are hardly inferior to those ofdiesel engines.

In an embodiment of the gas combustion engine in accordance with theinvention, the exhaust gas system and the air intake system are alsoadapted to application of gas as a fuel.

In a diesel engine, there is often a hollow in-the top surface of thepiston, on the bottom of which hollow is located a pointed protrusionserving to atomise the diesel fuel sprayed thereupon. A furtherembodiment of the gas combustion engine in accordance with the inventionis characterised in that the exhaust gas system and the air intakesystem are also adapted to application of gas as a fuel. In this manner,the combustion space is given a form which is advantageous to a gascombustion engine, whereby an homogeneous mixture of gas and airaccumulates in the hollow during operation. In addition, the compressionratio is hereby lowered, which is necessary when using gas as a fuel.

The enlarged hollow has such a diameter and depth that the ratio of thediameter to the depth lies between 30:9 and 30:13. Such a ratio ensuresgood heat extraction from the combustion-heated surfaces of the wall andbottom of the hollow. If this ratio is far removed from the statedrange, then that part of the piston located between the piston wall andthe wall of the hollow is unable to sufficiently impart its heat to therest of the piston, leading to damage to the piston, which isdetrimental to the lifetime of the engine.

In a preferential embodiment, this ratio is equal to 30:11. Inexperiments, this ratio transpired to be optimal with regard to heattransfer.

A further embodiment of the gas combustion engine in accordance with theinvention is characterised in that the wall of the piston is bevelled atthe extremity near to the top surface, whereby the external diameter ofthe piston decreases from a first value, remote from the top surface, toa second value, at the position of the top surface. As a result of theabove-elucidated heating of the walls of the hollow, arising fromcombustion of the fuel, that part of the piston located between thepiston wall and the wall of the hollow expands. This expansion is largerin proximity to the top surface of the piston than in proximity to thebottom surface of the hollow, due to better heat extraction near thebottom. As a result hereof, the diameter of the piston increases in thedirection of the top surface, causing the piston to press against thewall of the cylinder, as a consequence of which the piston wall eatsinto the cylinder wall, which can cause serious damage. However, as aresult of the bevelling hereabove referred to, there is room between thepiston wall and the cylinder wall, so that expansion of the piston wallwill not cause the piston wall to touch the cylinder wall.

An advantageous embodiment is characterised in that a number of pistonrings are located in the wall of the piston, and that the said bevellingextends as far as the top surface and begins at that piston ring whichis located nearest to the top surface.

Yet another embodiment of the gas combustion engine in accordance withthe invention is characterised in that the exhaust gas system comprisestwo exhaust branch-pieces, each of which is connected via a curved pipeto a separate catalyser, whereby the exhaust gas system is remotelylocated from an axial line of a cylinder such that the cylinder head isfreely accessible, when viewed in a transverse cross-section of theengine block. As a result hereof, one has access to the intake valves,exhaust valves and spark-plugs present in the cylinder head, withouthaving to remove the exhaust branch-pieces, or at least without beinghindered by the presence of the exhaust gas system. As is well known,diesel engines are not fitted with catalysers. The most obvious means ofincorporating a catalyser system would be to fit a single regulatedcatalyser in the existing exhaust gas system. However, an advantage ofthe current embodiment according to the invention is that, with twocatalysers of standard format, the same effect can be achieved as with asingle catalyser of larger format, and at reduced costs, since the costof two standard-format catalysers is lower than that of the alternativecatalyser of larger format. Moreover, as a result of splitting theexhaust gas system into two parts, the temperature is divided over bothparts, so that less substantial temperature-peaks occur, which has anadvantageous influence on the thermal loading of both the parts and thecatalysers.

An embodiment associated herewith is characterised in that a lambdaprobe is located in each pipe for the purpose of providing informationto an engine unit. As a result hereof, it is possible to obtain betterregulation of the operation of the catalysers.

In addition to the advantages already cited, which apply equally to themethod in accordance with the invention, an embodiment of the method inaccordance with the invention is characterised in that the volume of theintake branch-piece is reduced, and that a gas carburettor is coupled tothe intake branch-piece via a short pipe. During operation, the intakebranch-piece contains a fuel-air mixture, containing the air sucked inby the piston motion and a quantity of fuel injected by the carburettor.As a result of a reduced volume in the intake branch-piece, as well as asmall volume in the connection between the gas carburettor and theintake branch-piece, the consequences of a possible backfire (i.e. aspreading of flame from the cylinder to the intake branch-piece, via avalve opening) are reduced, due to the smaller quantity of mixture inthe intake branch-piece. In addition, there is a quickened reaction ofthe engine to a change in the position of the accelerator pedal, sincethe supply of old mixture having a composition corresponding to a formerposition of the accelerator pedal is more quickly depleted, allowing thenew mixture with the new composition to be obtained more quickly in thecylinders.

A further embodiment of the method in accordance with the invention ischaracterised in that atomiser openings in the cylinder head areenlarged in diameter, and that spark-plug mounting-bases are fitted inthese openings. As a result hereof, standard spark-plugs can beemployed, and the need for specially made spark-plugs (which wouldincrease the costs of the conversion) is removed.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention shall hereinafter be further elucidated on the basis of anexemplary embodiment as depicted in the drawing. To this end:

FIG. 1 renders a plan view of a gas combustion engine in accordance withthe invention,

FIG. 2 shows a longitudinal cross-section of the gas combustion enginedepicted in FIG. 1,

FIG. 3 gives a plan view of a piston of the gas combustion engine,

FIG. 4 depicts a cross-section along line A--A of the piston shown inFIG. 3,

FIG. 5 shows a detail of the piston cross-section contained in FIG. 4,

FIG. 6 gives a view of a cylinder head of the gas combustion engine, and

FIG. 7 renders a cross-sectional view of a valve cover of the gascombustion engine.

DETAILS OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a plan view of the gas combustion engine 1 in accordancewith the invention. This gas combustion engine is a converted dieselengine, in which inter alia the exhaust gas system 3 and the air intakesystem 5 are modified. In addition, the pistons and the cylinder headare modified, which, however, is not visible in this figure. The newexhaust gas system comprises two exhaust branch-pieces 7 and 9, whichare each connected to two openings in respective cylinder head halves 11and 13. The rightmost of both these openings is connected to an exhaustorifice of the rightmost cylinder, whereas the other opening isconnected via a cylinder head half to two exhaust openings of twoneighbouring cylinders. Both of the exhaust branch-pieces 7 and 9 arecoupled to two catalysers 19 and 21 via two curved pipes 15 and 17. Theexhaust pipes 23, 25 converge further up in the exhaust gas system 3.Lambda probes 27, 29 are located in the curved pipes 15, 17, whichprobes 27, 29 analyse the composition of the exhaust gases and providean engine regulation unit 31 with a corresponding signal. This engineregulation unit 31 subsequently emits signals which are used to regulatethe ignition-timing and the fuel supply.

The air intake system 5 comprises an intake branch-piece 33 which isconnected to a non-depicted gas carburettor via a short pipe 35. Thevolume of the intake branch-piece 33 is reduced with respect to that ofthe original intake branch-piece of the diesel engine.

Upon each of the cylinder head halves 11, 13 is located a valve cover37, 39, which can provide access to the intake and exhaust valves. Theentire exhaust gas system is located at a level above that of thepistons with cylinder head halves 11, 13 and valve cover 37, 39. As aresult hereof, the valve covers and cylinder head halves can be freelyaccessed, without experiencing hindrance from the exhaust gas system.

FIG. 2 schematically depicts a cross-section of the gas combustionengine 1, whereby the exhaust gas system 3 is also shown, despite thefact that this system 3 is located at a higher level than the drawncross-section. The exhaust branch-pieces 7, 9, which are located abovethe cylinder head 41 and engine block 43, are depicted with a dashedoutline. There are six cylinders 45 present in the cylinder block 43,which cylinders contain translatable pistons 47. The pistons areconnected to a crankshaft 51 via driving shafts 49. Each cylinder isconnected to the intake branch-piece 33 and an exhaust branch-piece 7, 9via an intake valve 53 and an exhaust valve 55, respectively. The volumeof the space 57 within the cylinder between the piston and the cylinderhead varies between a minimal value, in the uppermost position A of thepiston, and a maximal value, in the undermost position B of the piston.The compression ratio, which is defined as the ratio of the said minimalvalue to the said maximal value, is in this embodiment equal to 1:10. Inthe uppermost position of the piston, the said space forms a combustionspace 59. The combustion space 59 also extends into a hollow present inthe piston.

This figure also depicts spark-plug mounting-bases 61 via a dashedoutline.

FIGS. 3 and 4 respectively show a plan view and a cross-section of thepiston, in which the said hollow is clearly visible. The piston 47 has awall 63 and a top surface 65. The said hollow 67 is located in the topsurface 65, and has a diameter D and a depth H, whereby the ratio of thediameter to the depth is equal to 30:11. The hollow 67 is delimited by awall 69 and a flat bottom 71. Also located in the cylinder are twosemi-circular hollows 73 and 75, which respectively prevent the intakevalve and exhaust valve from coming into contact with the piston 47 inits uppermost positions. In the top surface 65 of the piston 67 is alsolocated a shallow, ring-like hollow 77, serving to optimise the form ofthe piston. All corner points 79, 81 and 83 are rounded off, so as toinhibit the possible formation of fine cracks. In the wall 63 of thepiston are located hollows in which piston rings 85, 87 are present.

FIG. 5 shows a detail of the piston 47, in proximity to the transitionfrom the top surface 65 into the wall 63. The piston wall 63 is bevellednear the top surface, which bevelling 89 extends from the top surface 65as far as the uppermost piston ring 87. As a result hereof, the diameterDz of the piston 47 decreases from a first value, remote from the topsurface, to a second value, at the position of the top surface. As aresult hereof, the expansion of the piston wall 91 between the wall 63and the wall 69 of the hollow, which is caused during operation by thecombustion of the exhaust gases in the combustion space, is compensatedfor. The difference between the first value and the second value amountsto a few tenths of a millimeter. The bevelling as it is depicted in FIG.5 is therefore strongly exaggerated, for purposes of clarity.

FIG. 6 depicts a cylinder head half 13 which is part of the cylinderhead, the latter being split into two halves. Spark-plugmounting-openings 93 are located in the cylinder head half 13, whichorifices 93 are formed by drill-widening the atomiser-openings alreadypresent in a diesel engine. The spark-plug mounting-bases 61, which areprovided with an extremity having external screw thread, can be fittedwithin these spark-plug mounting-openings 93, which are provided withinternal screw thread. In FIG. 7, a spark-plug mounting-base 61 of thistype is depicted via a dashed outline within the valve lid 39. Thespark-plug mounting-base 61 is provided at one end 95 with externalscrew thread, and is clamped near the other end by a ring 97 which islocated in an opening 99 in the valve cover39.

Furthermore, when converting a diesel engine to a gas combustion engine,the camshaft characteristic is altered in such a manner as to reduce thevalve overlap (i.e. the length of time for which both the intake valveand the exhaust valve are open).

I claim:
 1. Gas combustion engine comprising a gas exhaust system, anair intake system, an engine block comprising a number of cylinders andpistons, which pistons can be translated in the cylinders, and acylinder head which delimits a space in the cylinders at one extremity,whereby a volume of the space in a cylinder between the cylinder headand a piston can be varied between a minimal and a maximal value bymeans of a translatory motion of the piston, a ratio of the said minimaland maximal values defining a compression ratio, a combustion spacebeing defined by the space in that state of the piston which correspondsto a minimal volume, said gas combustion engine being a converted dieselengine, the combustion space and the compression ratio being adapted toapplication of gas as a fuel, wherein the pistons have a cylindricalwall and a top surface, a hollow being located in the top surface, whichhollow forms part of the combustion space and has an approximatelycircle-cylindrical form and a substantially flat bottom and the hollowhas a diameter and a depth, whereby the ratio of the diameter to thedepth lies between 30:9 and 30:13, said cylindrical wall of each pistonbeing bevelled near said top surface, an external diameter of each ofsaid pistons decreasing from a first value adjacent a piston ringclosest to said top surface to a second value at said top surface, saidpiston ring being one of a number of piston rings in the cylindricalwall of each of said pistons.
 2. Gas combustion engine according toclaim 1, wherein the exhaust gas system and the air intake system arealso adapted to application of gas as a fuel.
 3. Gas combustion engineaccording to claim 1, wherein said ratio of the diameter to the depth isapproximately equal to 30:11.
 4. Gas combustion engine according toclaim 1, wherein the exhaust gas system comprises two exhaustbranch-pieces, each of which is connected via a curved pipe to aseparate catalyser, the exhaust gas system being remotely located froman axial line of a cylinder such that the cylinder head is freelyaccessible, when viewed in a transverse cross-section of the engineblock.
 5. Gas combustion engine according to claim 4, further comprisinga lambda probe located in each of said pipes to provide information toan engine regulation unit.
 6. Method of manufacturing a gas combustionengine as claimed in claim 1, said method comprising the stepsof:enlarging a hollow in the top surface of the pistons both in diameterand in depth until a bottom of said hollow is approximately flat andsaid ratio of the diameter to the depth is obtained.
 7. Method accordingto claim 6, further comprising the steps of adapting the exhaust gassystem and the air intake system to an application of gas as a fuel. 8.Method according to claim 6, further comprising the step of bevellingthe wall of the piston at an end of the piston which neighbours the topsurface.
 9. Method according to claim 6, further comprising the step ofreplacing the exhaust gas system of the diesel engine by, inter alia,two exhaust branch-pieces, each of which is connected to a separatecatalyser.
 10. Method according to claim 6, further comprising the stepsof:reducing a volume of an intake branch-piece; and coupling a gascarburettor to the intake branch-piece via a short pipe.
 11. Methodaccording to claim 6, further comprising the steps of enlarging, indiameter, atomiser openings already present in the cylinder head;andfitting spark-plug mounting-bases in said enlarged atomiser openings.